Classical frequency synthesizers, which can be programmed to two or more frequencies, inherently have slow warm-up times during the reprogramming process. The same is true during initial power-up of the frequency synthesizer. The warm-up time for a programmable frequency synthesizer is considered the time taken by the frequency synthesizer to switch from one operating frequency to a second operating frequency within a desired frequency lock range (e.g., .+-.10 Hz from the desired second operating frequency), or the time period needed to achieve a desired power-up frequency within a desired frequency lock range. The slow warm-up time arises from the use of phase detectors which do not measure instantaneous phase such as, for example, the source-sink-float (SSF) phase detector. To compensate for the phase detector delay, other circuit parameters in the frequency synthesizer are adjusted in ways which complicate design as well as increase cost. In addition, phase detector delay affects the phase margin and gain bandwidth of the phase-locked loop in such a way as to extend the warm-up period.
Thus, what is needed is a method and apparatus for determining an instantaneous phase difference between two signals. Preferably, the method and apparatus must measure instantaneous phase while maintaining low power consumption.